This is a playground example inspired by a real task (much more complicated) that I once did. The basic flow is that there is the reading of records from a sequential file. Some of the records contain commands that require examining the previous records to calculate a value.
Undesirable about this solution is that it requires an extra list thus extra duplicate storage. That extra list is called REMEMBER in the following code. This example has a simple record structure just containing one data value so duplicating everything in the REMEMBER list is not a real issue. But please assume the real task involves a much more complicated record structure such that duplicating everything in the REMEMBER list is very undesirable.
I am inclined to use a doubly linked list however per discussion at this link Doubly Linked List in Prolog it seems that is not the Prolog way to do things. Therefore I am interested to see what the Prolog way of doing things should be.
/*
A file contains sequential records.
There are two types of record.
A data record provides a data value.
An average record provides a count and is a request for an average of the last count data values.
The parse dcg below parses a list from the data file.
The report dcg uses that list to generate a report.
After parse the list looks like this:
[value=5.9,value=4.7,value=7.5,average=3,value=9.0,value=1.1,value=8.3,average=5,value=7.1,value=1.3,value=6.7,value=9.9,value=0.5,value=0.3,value=1.5,value=0.2,average=7,value=2.2,value=7.8,value=2.5,value=4.5,value=2.4,value=9.7,average=4,value=5.2,value=8.5,value=2.2,value=8.0,value=0.7].
An example report looks like this:
[[count=3,total=18.1,average=6.033333333333333],[count=5,total=30.599999999999998,average=6.12],[count=7,total=20.400000000000002,average=2.9142857142857146],[count=4,total=19.1,average=4.775]].
*/
:- use_module(library(dcg/basics)).
:- use_module(library(readutil)).
:- use_module(library(clpfd)).
:- use_module(library(clpr)).
dospy
:-
spy(report),
spy(average),
leash(-all).
:- initialization main.
report(LIST)
-->
% the report starts with nothing to REMEMBER.
report(LIST,[]).
report([value=VALUE|LIST],REMEMBER)
-->
% value in the LIST goes into REMEMBER.
report(LIST,[value=VALUE|REMEMBER]).
report([average=COUNT|LIST],REMEMBER)
-->
% request for average in the LIST.
average(REMEMBER,COUNT),
report(LIST,REMEMBER).
report([],_REMEMBER)
% the LIST is empty so the report is done.
--> [].
average(REMEMBER,COUNT)
-->
% the average starts at 0 then accumulates for COUNT values from REMEMBER.
average(REMEMBER,COUNT,0,0.0).
average([value=VALUE|REMEMBER],COUNT,AT,TOTAL)
-->
% found needed value in the REMEMBER.
clpfd( AT #\= COUNT ),
clpfd( AT_NEXT #= AT + 1 ),
clpr( TOTAL_NEXT = TOTAL + VALUE ),
average(REMEMBER,COUNT,AT_NEXT,TOTAL_NEXT).
average(_REMEMBER,COUNT,COUNT,TOTAL)
-->
% all of the needed value have been seen so calculate and add to report.
clpr( AVERAGE = TOTAL / COUNT ),
[[count=COUNT,total=TOTAL,average=AVERAGE]].
% now the part that does the parse of the data file.
parse(LIST) --> parse(data,LIST).
parse(LIST) --> parse(average,LIST).
parse(LIST) --> parse(end,LIST).
parse(data,[value=FLOAT|LIST])
-->
"data", whites, float(FLOAT), blanks, !,
parse(LIST).
parse(average,[average=COUNT|LIST])
-->
"average", whites, integer(COUNT), blanks, !,
parse(LIST).
parse(end,[])
-->
[].
clpr( CLPR )
-->
{ clpr:{ CLPR } }.
clpfd( CLPFD )
-->
{ CLPFD }.
main :-
system:( read_file_to_codes("doubly_motivation_data.txt",CODES,[]) ),
prolog:( phrase(parse(LIST),CODES) ),
system:( writeq(LIST),writeln(.) ),
prolog:( phrase(report(LIST),REPORT) ),
system:( writeq(REPORT),writeln(.) ).
/* doubly_motivation_data.txt
data 5.9
data 4.7
data 7.5
average 3
data 9.0
data 1.1
data 8.3
average 5
data 7.1
data 1.3
data 6.7
data 9.9
data 0.5
data 0.3
data 1.5
data 0.2
average 7
data 2.2
data 7.8
data 2.5
data 4.5
data 2.4
data 9.7
average 4
data 5.2
data 8.5
data 2.2
data 8.0
data 0.7
*/
